A speaker is a transducer that converts electrical energy into sound power for vibrating a film to generate a sound according to an electrical signal. According to driving manners of different speakers, the speakers can be classified into dynamic, piezoelectric, and electrostatic types of speakers.
Nowadays the dynamic speakers have been widely used and applied to products such as televisions, stereograms, microphones, and mobile phones. However, the dynamic speakers are limited by their inherent structures and electrical energy designs. They can not serve the needs of human beings in pursuance of portable and low-current electrical products. The piezoelectric speaker utilizes a piezoelectric effect to deform a piezoelectric material to drive a diaphragm for releasing a sound. However, the resonant frequency of the piezoelectric material is high. Therefore, the piezoelectric speaker is merely applied to an alarm buzzer for now.
The operational principle of the electrostatic speaker is described as follows. A conductive vibrating film is disposed between two fixed electrode plates to form a condenser. A DC bias voltage is applied to the vibrating film and AC voltages of opposite phases are applied to the two electrode plates. The electrostatic force generated by positive and negative charges therebetween drives the vibrating film to vibrate to generate and release sound. A conventional electrostatic speaker requires an amplifier to supply a high DC bias voltage (1,500V-2,000V). However, the conventional electrostatic speaker is expensive and may not be suitable for portable products.
Electret speakers are newly developed, and they are a type of electrostatic speaker. The electret speaker has a flexible characteristic, so it is often referred to as a flexible speaker. The electret speaker has advantages of light weight and small in size, high efficiency, high bandwidth, and low distortion. Considering the size and efficiency, the electret speaker has a greater potential for portable products.
However, the conventional electrostatic speaker and the electret speaker still have many problems to overcome. For example, electrostatic charges may run off since the vibrating film may touch the conductive backplate during a vibrating period or a non-vibrating period of the vibrating film. For this reason, the lifespan of the speaker is decreased. If the stability problem of the electrostatic speaker is not solved, its application in use will be significantly limited.
U.S. Pat. No. 3,646,280 discloses an electrostatic speaker utilizing a conductive fiber for serving as an electrode. Since the conductive fiber directly touches an electrostatic vibrating film (or an electret film), electrostatic charges of the vibrating film may easily run off. US Patent Publication Serial No. 20090016552 discloses an electrostatic speaker utilizing a porous conductive layer for serving as an electrode. The electrode is unprotected so the lifespan of an electrostatic film will be decreased.
Taiwan Patent Serial No. I293233 discloses an electret speaker utilizing a flexible conductive plate or a metal mesh for serving as an electrode. The electrode may contact a vibrating film so electret charges of the vibrating film may easily run off. Taiwan Patent Serial No. I294250 discloses an electret actuator utilizing a conductive plate forming a plurality of holes or apertures for serving as an electrode. The electrode is unprotected so the lifespan of the electret actuator will be decreased.
Therefore, how to improve the stability of an electrostatic speaker and increase the lifespan of the speaker is an important issue in this technical field.